The drivers carpet was damp. I decided to take the wing off to have a look.
View attachment 48175



:bang:
My heart goes out to you.
That looks like in-built rust to me - as though the steel was lightly pre-rusted before assembly. It reminds me of a Mk2 door I saw once where the frame had totally disappeared, yet the door skin (hanging free) was completely un-rusted. My guess is that the frame steel was pre-rusted... and similarly in your case the rust seems to run right past other steel without the other steel being affected!
On Trademe, there's a white Mk2 Selecta. It looks almost perfect (there is some just-noticeable rust at the top of one rear door, under the window, but not bubbles and not through the paint, just some orange marks, an easy fix). Otherwise it IS perfect, all white paint matches and no dents anywhere, even the bumpers are nice and black, the interior pristine, only done 70,000km, engine (1372cc) and transmission in full working condition...
...and lapsed registration. Apparently the owners forgot to relicense it when they went overseas, and didn't know about getting an exemption either
The guy that owns it has been advertising it as parts, but the car is far too good to wreck - and we both agreed that. So far, he's sold parts off another Uno instead. The white Selecta has a modest asking price - well under four figures - that's all I'm going to say. PM me if you'd like to discuss further!
As James says, that rust could be fixed - but yuck, what a job when it goes behind the under-seat crossmember and down to the floor. And, what are the odds of there being rust under the floor as well - jacking points? How about the rear strut turrets?
Also I'm a bit confused, as I thought you had a Mk1 70?
-Alex